318 K. KUEDEMANN MODE OF LIFE OF PRIMITIVE CEPHALOl'ODS 



with Avide-open apertures. According to the usual procedure, these 

 would be distinguished as different species, the first two as belonging to 

 Oncoceras, the third as a Cyrtoceras. 



The fact that both the larger and smaller forms with contracted aper- 

 tures exhibit more closely arranged septa just below the living chamber 

 indicates that the contracted apertures actually indicate a gerontic con- 

 dition, and that both forms, the larger and the smaller, had reached or 

 passed maturity. Outside of this difference in size, the two forms agree 

 absolutely in all other diagnostic characters, as relative rate of growth, 

 depth of camerse and septa; and likewise does the third form, with wide-. 



I'-KiuuE 4. — Oncoceras 

 irupwformc nov. 



Mature female. Natural 

 size. 



FiGUUE 5. — Oncoceras 

 pupa'forme nov. 



Mature male. Natural 

 size. 



FiGUitE 6. — Oncoceras 

 pupwforiite nov. 



Immature female. Natural 

 size. 



open aperture, fail to show any other differences suggestive of specific 

 distinction. This third form is also of smaller size and ranges between 

 the smaller and larger forms. 



We consider these three forms as belonging to a single species of 

 Oncoceras (to be described in another place), the larger and smaller 

 forms, with contracted apertures, representing the matxire females and 

 males respectively, and the intermediate form, with uncontracted aper- 

 ture, the immature females. 



The conclusion that the larger shells contained the females of the spe- 

 cies and the smaller ones the males is based on the observation in recent 

 cephalopods that, as a rule, the males are more slender or smaller than 

 the females. In Argonouia, where the maximum of sexual dimorphism 

 is found, the females are as much as 15 times as long as the males. 



